Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, fuel cells have been identified as a potential alternative for the traditional internal-combustion engine used in modern automobiles.
A fuel cell typically includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, generally platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrolyte-assembly (MEA). The MEA is typically disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants such as hydrogen and oxygen to the cathode and anode. Hydrogen at the anode is converted to positively-charged hydrogen ions. These ions travel through the electrolyte to the cathode, where they react with oxygen. The oxygen can be supplied from air, for example. The remaining electrons in the anode flow through an external circuit to the cathode, where they join the oxygen and the hydrogen protons to form water.
Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to power a vehicle. During a typical start-up operation of the fuel cell stack, hydrogen enters and flows through the individual fuel cells from one end of the fuel cell stack to another. The ends of the fuel cell stack are often referred to as the wet and dry ends, with the hydrogen generally flowing from the wet end to the dry end. In this manner, hydrogen is distributed to the individual fuel cells in a sequential fashion, with delivery of the hydrogen to a portion of the fuel cells adjacent the wet end occurring first and delivery of the hydrogen to a portion of the fuel cells adjacent the dry end occurring last. Thus, the fuel cells at the dry end often receive hydrogen at a point in time after the fuel cells at the wet end receive hydrogen. In an alternate arrangement, hydrogen may also be fed from the dry end for regular operation and sometimes from both ends during startup. In all these cases there is a non-uniform distribution of hydrogen.
It is also well known in the art that high cell voltage combined with a hydrogen-air front passing through the cell results in cell voltage degradation induced by carbon corrosion. When hydrogen is received by the anodes of the fuel cells, the hydrogen replaces the air in the anodes and creates the hydrogen-air front that travels along a length of the anodes. In order to mitigate carbon corrosion during startup, a resistive load is typically placed across the stack to suppress cell voltage, thus reducing carbon corrosion. The lower the resistance, the lower the voltage during the hydrogen-air front.
Unfortunately, non-uniform distribution of hydrogen to the fuel cells during the start-up operation and the use of a resistive load can lead to a severe performance degradation of fuel cell stack. The presence of air on the cathodes coupled with the hydrogen-air front on the anodes can cause an undesirable electric potential to form. In particular, the non-uniform distribution of hydrogen on the anodes of the fuel cell stack can lead to a high potential phenomenon known as “cell reversal.” Cell reversal occurs when a load is applied to the fuel cell stack and when at least one fuel cell in the fuel cell stack lacks hydrogen while other fuel cells in the fuel cell stack are supplied with hydrogen. Cell reversal causes an oxidation of fuel cell components and may result in a rapid voltage degradation of the fuel cell stack. In particular, a corrosion of the carbon substrate of the electrodes, wherein surface oxides, CO, and CO2 are formed, occurs. The voltage degradation significantly reduces the useful life of the fuel cell stack.
A number of fuel cell systems and methods are known in the art for optimizing the distribution of hydrogen to the anodes of the fuel cell stack. For example, it is known in the art to purge the anodes of the fuel cell stack with hydrogen at system start-up so as to minimize a time that the hydrogen-air front exists in the flow channels of the anodes. It is also known to cause hydrogen to flow from both the first end and the second end of the fuel cell stack to optimize a distribution of hydrogen to the anode cells along a length of a fuel cell stack. Such a method has been found to result in the non-uniform distribution of hydrogen at the fuel cells disposed at the center of the fuel cell stack, however.
Geschwindt, et al., in U.S. Pub. Pat. App. No. 2005/0129999 reports an inlet fuel distributor having a permeable baffle disposed between a fuel supply pipe and a fuel inlet manifold, causing hydrogen to be uniformly distributed along the length of the fuel inlet manifold. During start-up, hydrogen or inert gas within the inlet fuel distributor and the fuel inlet manifold may be vented through a valve in response to a controller to present a uniform hydrogen front to the inlets of the fuel flow fields.
An independent supply of hydrogen to the anode cells has also been employed to minimize voltage degradation. For example, in U.S. Pub. Pat. App. No. 2005/0118487 to Whiton, et al., a fuel cell stack having an inlet fuel distributor including a plurality of conduits of substantially equal length and equal flow cross section is described. The fuel cell stack is reported to uniformly distribute fuel cell inlet fuel from a fuel supply pipe to a fuel inlet manifold. The conduits may be either channels formed within a plate or tubes and may have single exits or double exits into the fuel inlet manifold.
There is a continuing need for a fuel cell system and method that optimizes a distribution of hydrogen during a start-up operation and militates against a voltage degradation of the fuel cell stack. Desirably, the fuel cell stack and method also minimizes hydrogen exhaust emissions during the start-up operation.